gdb: cleanup command creation in infcmd.c

gdb: cleanup command creation in infcmd.c

In infcmd.c, in order to add command completion to some of the 'set'
commands, we are currently creating the command, then looking up the
command by calling lookup_cmd.

This is no longer necessary, we already return the relevant
cmd_list_element object when the set/show command is created, and we
can use that to set the command completion callback.

I don't know if there's actually any tests for completion of these
commands, but I manually checked, and each command still appears to
offer the expected filename completion.

There should be no user visible changes after this commit.

Reviewed-By: Tom Tromey <tom@tromey.com>